The present invention relates in general to semiconductor memories and, more particularly, to a non-volatile semiconductor memory made with ferro-electric capacitors.
Semiconductor memories are used in a myriad of applications to store data for later retrieval and usage. Many applications require the memory to be non-volatile in that the data must remain valid for a long period of time even when external power is removed from the memory cell. One such application involves the use of tag memory where one or more memory cells are placed on an item, e.g. personal luggage, that does not have a power source. The tag memory on the luggage may be accessed, for example at airports and depots, to make identification as to the owner and destination. An RF signal is transmitted remotely from a reader to access the tag memory. Some of the received RF transmission signal power energizes the tag memory cell so that the data may be retrieved and transmitted back to the reader. The tag memory must be non-volatile since it remains dormant for long periods of time between accesses.
In the prior art, ferro-electric capacitors have been used as the non-volatile memory storage element. However, the number of available data accesses is limited because the ferro-electric property degrades with each memory access cycle. Most if not all ferro-electric memory structures drive the ferro-electric material completely around its hysteresis loop every time the ferro-electric memory cell is accessed. Thus, with the ferro-electric capacitors used as the non-volatile memory storage element, the continuous memory cycling leads to an unnecessary amount of polarization domain switching that also causes excessive power consumption by the ferro-electric cell and reduces the endurance and data retention time of the memory circuit.
Hence, a need exists for a non-volatile storage device that can be accessed remotely while retaining data for long periods of time.